Are you working on a project you started on PP CC14, or is it a new project created in CC15?
Is your project using any special effects, plug-ins etc? And did you make sure the sequence settings match the source footage?

Try clearing your media cache manually:
1. inside PP navigate to Preferences > Media to see where your cache and cache database are saved
2. quit Premiere and navigate to these folders in the finder
3. delete all the files, relaunch PP and let it recreate the cache files.

Also, what graphic card do you have? Do you have the CUDA drivers installed? Are you using GPU acceleration at all (CUDA / Open CL)? And if you are, what happens if you switch to software-only?
Some people have reported CUDA issues after updating to CC15, but removing cache and updating the drivers worked for most.